This invention relates to an ultrasonic flaw detector, and particularly to a flaw detector used in connection with the detection of fatigue cracks associated with fasteners in aircraft wing surfaces.
Fatique cracks extending from openings in metal surfaces, such as those openings formed to accept fasteners in aircraft wings, will grow when subject to stress. If cracks and other flaws associated with these fasteners in aircraft wings can be detected while they are yet very small, the fastener can be removed, the hole enlarged to remove the flaw, and an oversized fastener installed in its place.
Since fasteners are available in 1/64 inch increments, it would be desirable if fatigue cracks within this size range could be detected. However, cracks of this size are difficult to detect with prior art techniques without also experiencing an unreasonably large false call rate, that is, the detection of apparent cracks or flaws caused by something other than an actual crack or flaw, such as out-of-round holes.
If the crack becomes as large as 0.200 inch, catastrophic failure of the wing could occur. It is not unusual for the prior art devices to reliably detect cracks as small as 0.030 inch, however, detecting cracks as small as 0.010 inch requires the special techniques developed in connection with this invention.
Those experienced with the operation of prior art devices, for example that shown in U.S. Pat. No. 3,977,236, have noted that as an ultrasonic transducer is rotated around the central axis of a fastener, an A-scan presentation of the crack or flaw return signal will apparently move as the transducer is rotated. This moving signal, however, is sometimes masked by other return signals, and therefore very small cracks are frequently undetected.